This invention relates to a process for treating gases contaminated with very minor amounts of oxygen. More particularly it is adapted to treating substantial flows of gaseous olefins and inert gases, and selectively reducing oxygen levels from a few parts per million by weight (ppmw) to levels less than about 0.3 ppmw. The treatment is accomplished by contacting the gas contaminated with oxygen as feed with a bed of certain coal derived activated carbon, under certain conditions, and separating from the bed a product gas having a reduced oxygen content.
Inert gas i.e. nearly oxygen-free gas is used in many industrial applications, such as e.g., for purging, blanketing, and for maintaining an inert atmosphere in material transport. The gas may be manufactured by stoichiometric combustion of hydrocarbon fuel in which case it contains carbon dioxide, water vapor, nitrogen and traces of oxygen, carbon monoxide, hydrogen and rare inert atmospheric gases such as argon; or cryogenically, in which case it primarily contains nitrogen, and traces of oxygen.
Particularly in the preparation of high molecular weight polymers, free oxygen must be excluded as much as possible from feed gases employed for many catalytic polymerization processes. Oxygen may be present as a contaminant in e.g. olefinic hydrocarbons as the result of the manufacturing process and/or during storage. For the polymerization of for example, ethylene and/or propylene, the presence of very minor amounts i.e. more than a few tenths of a part per million by weight, of oxygen may rapidly deactivate the catalyst, or otherwise adversely impact properties of the resulting polymer.
Various methods have been proposed for sorbing trace amounts of oxygen from these gaes. Japanese Pat. No. 061769 discloses the use of high molecular weight complexes of certain low molecular weight complexes of transition metal of groups VIA, VIII, IB and IIB of the fourth period of the periodic table, which are ligand substituted with water soluble high molecular weight ligands containing: (a) radicals having coordinating ability to the base metal, such as e.g., an amine radical, and (b) longchain alkyl radicals, such as e.g. dodecyl-, situated in the vicinity of radicals (a).
German Pat. No. 2,553,567 discloses that trace amounts of oxygen and other impurities may be sorbed from olefins and inert gases using sorbents prepared by impregnating a carrier material having a specific surface area of at least 1200 m.sup.2 /g and a specific pore volume of at least 0.5 cm.sup.3 /g with an aqueous solution of a manganese compound, activating at 60.degree. to 600.degree. C. under a pressure between 10 mm. of mercury and 5 atmospheres in an oxygen containing gas for for 5-300 minutes and treating the activated material with a reducing gas under a pressure between 10 mm of mercury and 5 atmospheres at 250.degree.-600.degree. C. for 1 to 60 minutes. Suitable carriers are silica gel, titanium dioxide, thorium oxide, zirconium oxide, magnesium oxide, magnesium hydroxide, magnesium carbonate, magnesium alcoholates, zeolites, or silica gel with a magnesium oxide surface layer.
Russian Pat. No. 706,387 discloses the use of an oxygen sorbent that contains 0.5-3% W Cr.sub.2 O.sub.3 on silica gel.
Activated carbon is a well known gas and vapor sorbent. It is used to remove corrosive gases and vapors from intake air to protect equipment in telephone exchanges, and is also used to lengthen the storage life of apples in warehouses by removing ethylene from the atmosphere. Further it is known that wet activated carbon depletes oxygen from air; that workers entering a vessel containing carbon must take precautions since dangerously low levels of oxygen may be encountered. It has now been found that certain activated carbon may be used to selectively reduce even trace amounts of oxygen present in inert gases, and in polymerizable aliphatic olefin gases such as ethylene and propylene.
It is known from Johannes, C. Advances in Cryogenic Engineering, Vol. 17, pp 307-12 that activated carbon derived from cocoanut charcoal can reduce oxygen in Helium at 70.degree. K. to levels of about 1.3 ppm. However, the use of such low temperatures is not practical for many industrial applications.